Production of metal castings with an internal void space is commonly achieved by including a resin-bonded sand structure, called a sand core, which has the shape of the desired void space and is suspended at the desired location within the casting cavity prior to metal fill. As molten metal enters the mold cavity (for example, a sand mold cavity) it flows around the sand core and begins its solidification in the forming of an engine block or other cast article. The heat of the metal is intended to decompose the binder of the sand core after a solid cast skin has formed against the core to duplicate the shape of the core.
Organic-based materials are commonly used as binders for the sand particles in sand cores for the explicit purpose of undergoing thermal degradation to allow removal of the sand particles from the solidified casting by mechanical shaking. Cast iron alloys are often poured at temperatures in excess of 1000° C. but aluminum alloys are often poured around 700° C. The temperatures experienced by the cores may be a few hundred degrees lower. Failure to achieve sufficient degradation, often encountered when casting aluminum alloys, can make the shake-out of sand core material very difficult to complete. This results in the need to employ further time-consuming and costly processes such as a prolonged heat treatment and/or very intensive mechanical impacting and shaking to disaggregate the interior cores.
Polyurethane polymers are currently a commonly used core binder material in automotive vehicle manufacturer foundry operations owing to their good bonding strength and rapid molding cycle times when using the gas-catalyzed process, referred to as a “cold box” method. The gaseous catalyst for this process is a volatile organic base such as triethylamine. Also, there are similar polyurethane binder systems being employed which use a liquid amine catalyst and are called “no-bake” processes. The basic polymer chemistry is the same for both methods involving the reaction of an isocyanate prepolymer with a polyol when exposed to an amine catalyst. The isocyanate component in all systems currently in use is an oligomeric form of MDI, methylene diphenyl diisocyanate. Various polyols are employed by different manufacturers, with a phenol-formaldehyde pre-resin as a component for the cold box method.
Because of the shakeout problems encountered when casting aluminum or other low-temperature melting metals, it is common practice to limit the resin content in the sand. This, however, places a limit on the strength of the core which becomes a significant disadvantage when attempting to employ very thin or elongated core geometries that can distort and lose dimensions due to softening during the casting process. Other efforts to create polyurethane core binders with better shake out capability have included chemical modifications to the polymer structure.
There remains a need for an improved practice for facilitating the timely chemical decomposition of polyurethane resin binder materials in sand cores to permit easy removal of the sand particles from a metal casting. The need is particularly acute in the manufacture of complex castings of metal alloys such as aluminum alloys and magnesium alloys and the like. Aluminum engine parts and other drivetrain parts often require the use of one or more sand cores in each casting and efficient production of such parts requires easy shakeout of the sand from each core from the solidified cast article.